1. Field of the Invention
This invention relates to data processing systems and, more particularly, to data processing systems wherein data items are identified by substantially unique identifiers which depend on all of the data in the data items and only on the data in the data items.
2. Background of the Invention
Data processing (DP) systems, computers, networks of computers, or the like, typically offer users and programs various ways to identify the data in the systems.
Users typically identify data in the data processing system by giving the data some form of name. For example, a typical operating system (OS) on a computer provides a file system in which data items are named by alphanumeric identifiers. Programs typically identify data in the data processing system using a location or address. For example, a program may identify a record in a file or database by using a record number which serves to locate that record.
In all but the most primitive operating systems, users and programs are able to create and use collections of named data items, these collections themselves being named by identifiers. These named collections can then, themselves, be made part of other named collections. For example, an OS may provide mechanisms to group files (data items) into directories (collections). These directories can then, themselves be made part of other directories. A data item may thus be identified relative to these nested directories using a sequence of names, or a so-called pathname, which defines a path through the directories to a particular data item (file or directory).
As another example, a database management system may group data records (data items) into tables and then group these tables into database files (collections). The complete address of any data record can then be specified using the database file name, the table name, and the record number of that data record.
Other examples of identifying data items include: identifying files in a network file system, identifying objects in an object-oriented database, identifying images in an image database, and identifying articles in a text database.
In general, the terms "data" and "data item" as used herein refer to sequences of bits. Thus a data item may be the contents of a file, a portion of a file, a page in memory, an object in an object-oriented program, a digital message, a digital scanned image, a part of a video or audio signal, or any other entity which can be represented by a sequence of bits. The term "data processing" herein refers to the processing of data items, and is sometimes dependent on the type of data item being processed. For example, a data processor for a digital image may differ from a data processor for an audio signal.
In all of the prior data processing systems the names or identifiers provided to identify data items (the data items being files, directories, records in the database, objects in object-oriented programming, locations in memory or on a physical device, or the like) are always defined relative to a specific context. For instance, the file identified by a particular file name can only be determined when the directory containing the file (the context) is known. The file identified by a pathname can be determined only when the file system (context) is known. Similarly, the addresses in a process address space, the keys in a database table, or domain names on a global computer network such as the Internet are meaningful only because they are specified relative to a context.
In prior art systems for identifying data items there is no direct relationship between the data names and the data item. The same data name in two different contexts may refer to different data items, and two different data names in the same context may refer to the same data item.
In addition, because there is no correlation between a data name and the data it refers to, there is no a priori way to confirm that a given data item is in fact the one named by a data name. For instance, in a DP system, if one processor requests that another processor deliver a data item with a given data name, the requesting processor cannot, in general, verify that the data delivered is the correct data (given only the name). Therefore it may require further processing, typically on the part of the requestor, to verify that the data item it has obtained is, in fact, the item it requested.
A common operation in a DP system is adding a new data item to the system. When a new data item is added to the system, a name can be assigned to it only by updating the context in which names are defined. Thus such systems require a centralized mechanism for the management of names. Such a mechanism is required even in a multi-processing system when data items are created and identified at separate processors in distinct locations, and in which there is no other need for communication when data items are added.
In many data processing systems or environments, data items are transferred between different locations in the system. These locations may be processors in the data processing system, storage devices, memory, or the like. For example, one processor may obtain a data item from another processor or from an external storage device, such as a floppy disk, and may incorporate that data item into its system (using the name provided with that data item).
However, when a processor (or some location) obtains a data item from another location in the DP system, it is possible that this obtained data item is already present in the system (either at the location of the processor or at some other location accessible by the processor) and therefore a duplicate of the data item is created. This situation is common in a network data processing environment where proprietary software products are installed from floppy disks onto several processors sharing a common file server. In these systems, it is often the case that the same product will be installed on several systems, so that several copies of each file will reside on the common file server.
In some data processing systems in which several processors are connected in a network, one system is designated as a cache server to maintain master copies of data items, and other systems are designated as cache clients to copy local copies of the master data items into a local cache on an as-needed basis. Before using a cached item, a cache client must either reload the cached item, be informed of changes to the cached item, or confirm that the master item corresponding to the cached item has not changed. In other words, a cache client must synchronize its data items with those on the cache server. This synchronization may involve reloading data items onto the cache client. The need to keep the cache synchronized or reload it adds significant overhead to existing caching mechanisms.
In view of the above and other problems with prior art systems, it is therefore desirable to have a mechanism which allows each processor in a multiprocessor system to determine a common and substantially unique identifier for a data item, using only the data in the data item and not relying on any sort of context.
It is further desirable to have a mechanism for reducing multiple copies of data items in a data processing system and to have a mechanism which enables the identification of identical data items so as to reduce multiple copies. It is further desirable to determine whether two instances of a data item are in fact the same data item, and to perform various other systems' functions and applications on data items without relying on any context information or properties of the data item.
It is also desirable to provide such a mechanism in such a way as to make it transparent to users of the data processing system, and it is desirable that a single mechanism be used to address each of the problems described above.